Chambered mufflers like your Flows will always be loud. You want a straight thru design like Dynomax Ultra Flo's, Magnaflow, Borla and the like. Shy away from MAC, Flowmaster, Spintech and such since they are noise makers.
With an off road h pipe (assuming thats what you have) all mufflers will be pretty loud, I have also heard that the super turbos from dynomax are the quietest.
I have had about five different types of mufflers on my car to see what not only sounded the best but also performed the best as well. I settled on the dynomax ultra flo welded muffler. The muffler is somewhat loud outside my car but hardly noticeable inside the car. It has a great sound and I get asked about my exhaust all the time since it is not a common thing around here to see dynomax on a mustang. Everybody here runs flowmaster or a select few dished out big $ for bassani.
I've heard that the hooker maxflows are supposed to be really quiet yet still flow really really well. I know they are popular with the turbo boys...so they must perform well since the exhaust is critical to their performance.